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FOR IMMEDIATE RELEASE

PROSPERITY BANCSHARES, INC.®

REPORTS FOURTH QUARTER

2025 EARNINGS

HOUSTON, January 28, 2026. Prosperity Bancshares, Inc.® (NYSE: PB) (“Prosperity Bancshares”), the parent company of Prosperity Bank® (collectively, “Prosperity”), reported net income of $139.9 million for the quarter ended December 31, 2025, compared with $130.1 million for the same period in 2024. Net income per diluted common share was $1.49 for the quarter ended December 31, 2025, compared with $1.37 for the same period in 2024. The annualized return on fourth quarter average assets was 1.49%. Additionally, deposits increased $700.4 million during the fourth quarter of 2025. Nonperforming assets remain low at 0.46% of fourth quarter average interest-earning assets. On January 1, 2026, American Bank Holding Corporation (“American”) merged with Prosperity Bancshares and American Bank, N.A. (“American Bank”) merged with Prosperity Bank (collectively, the “Prosperity/American Merger”).

“I am excited to announce that on January 1, 2026, Prosperity completed the merger with our new partner American and its wholly owned subsidiary American Bank, headquartered in Corpus Christi, Texas. In connection with that transaction, we are pleased that Patt Wallace, the daughter of one of the founding families of the bank, and Steve Rafaelle, the CEO of American Bank, have joined our Bank Board of Directors,” said David Zalman, Prosperity’s Senior Chairman and Chief Executive Officer.

“We have also received all regulatory and shareholder approvals for the merger with Southwest Bancshares, the parent company of Texas Partners Bank and expect the transaction will be effective on February 1, 2026. In connection with the Southwest deal, we are pleased that Gene Dawson, Interim Chairman of Southwest Bancshares and Chairman of the nationally recognized Pape-Dawson engineering firm will be joining our Bank Board of Directors. To further add to our San Antonio presence, Charlie Amato has joined our Bank Board of Directors. In addition to his successful business, Charlie previously served as a board member of Federal Reserve Board of Dallas, San Antonio Branch, a Regent of the Texas State University System and is an investor in the San Antonio Spurs,” continued Zalman.

“When Prosperity went public in 1998, we were a small community bank in rural Texas with less than $500 million in assets. For 27 years, we have remained disciplined and focused on the same strategy. Delivering shareholder value by prioritizing low-cost core deposits, operational efficiency, sound credit quality, and growth via opportunistic M&A,” added Zalman.

“This morning’s announcement that Prosperity is acquiring Stellar Bancorp is consistent with that strategy and this transaction marks an important milestone for the company. Our combined Houston bank deposit rank increases from number 9 to number 5, making us the largest Texas-based bank in the market and 2nd largest by bank deposits in the state,” stated Zalman.

“Importantly, Stellar Bancorp is a well-run bank with similar credit discipline and an envious noninterest-bearing deposit mix. It has scarcity value, a quality balance sheet and earnings power. As a result, we view the transaction as a low-risk combination that significantly enhances our Texas footprint,” concluded Zalman.

Results of Operations for the Three Months Ended December 31, 2025

Net income was $139.9 million(2) for the three months ended December 31, 2025, compared with $130.1 million(3) for the same period in 2024, an increase of $9.8 million or 7.6%. Net income per diluted common share was $1.49 for the three months ended December 31, 2025, compared with $1.37 for the same period in 2024, an increase of 8.8%. The changes were primarily due to an increase in net interest income and a decrease in Federal Deposit Insurance Corporation (“FDIC”) special assessment, partially offset by an increase in provision for income taxes. On a linked quarter basis, net income was $139.9 million(2) for the three months ended December 31, 2025, compared with $137.6 million(4) for the three months ended September 30, 2025, an increase of $2.4 million or 1.7%. Net income per diluted common share was $1.49 for the three months ended December 31, 2025, compared with $1.45 for the three months ended September 30, 2025, an increase of 2.8%. Annualized returns on average assets, average common equity and average tangible common equity for the three months ended December 31, 2025, were 1.49%, 7.30% and 13.61%(1), respectively. Prosperity’s efficiency ratio (excluding net gains and losses on the sale, write-down or write-up of assets and securities) was 43.66%(1) for the three months ended December 31, 2025.

Net interest income before provision for credit losses was $275.0 million for the three months ended December 31, 2025, compared with $267.8 million for the same period in 2024, an increase of $7.2 million or 2.7%. The net interest margin on a tax equivalent basis was 3.30% for the three months ended December 31, 2025, compared with 3.05% for the same period in 2024. The changes to both measures were primarily due to a decrease in the average balances and average rates on other borrowings and a decrease in the average rates on interest-bearing deposits, partially offset by a decrease in the average balances and average rates on loans and federal funds sold and other earning assets. Net interest income before provision for credit losses increased $1.5 million to $275.0 million for the three months ended December 31, 2025, compared with $273.4 million for the three months ended September 30, 2025. The net interest margin on a tax equivalent basis was 3.30% for the three months ended December 31, 2025, compared with 3.24% for the three months ended September 30, 2025. The changes to both measures were primarily due to a decrease in the average balances and average rates on other borrowings, partially offset by a decrease in the average balances and average rates on loans and federal funds sold and other earning assets.

Noninterest income was $42.8 million for the three months ended December 31, 2025, compared with $39.8 million for the same period in 2024, an increase of $2.9 million, primarily due to increases in other noninterest income and service charges on deposit

accounts. Noninterest income was $42.8 million for the three months ended December 31, 2025, compared with $41.2 million for the three months ended September 30, 2025, an increase of $1.5 million.

Noninterest expense was $138.7 million for the three months ended December 31, 2025, compared with $141.5 million for the same period in 2024, a decrease of $2.8 million, primarily due to a reversal of the 2024 FDIC special assessment, partially offset by a change in the net loss on sale or write-down of other real estate and an excise tax expense due to stock repurchases. Noninterest expense was $138.7 million for the three months ended December 31, 2025, and $138.6 million for the three months ended September 30, 2025.

Results of Operations for the Year Ended December 31, 2025

For the year ended December 31, 2025, net income was $542.8 million(5) compared with $479.4 million(6) for the same period in 2024, an increase of $63.5 million or 13.2%. Net income per diluted common share was $5.72 for the year ended December 31, 2025, compared with $5.05 for the same period in 2024, an increase of 13.3%. The changes were primarily due to an increase in net interest income, lower merger related provision and expenses, and lower regulatory assessments and FDIC insurance, partially offset by a decrease in net gain on sale or write-up of securities. Returns on average assets, average common equity and average tangible common equity for the year ended December 31, 2025, were 1.42%, 7.14% and 13.43%(1), respectively.

Net interest income before provision for credit losses for the year ended December 31, 2025, was $1.081 billion compared with $1.026 billion for the same period in 2024, an increase of $55.0 million or 5.4%. The net interest margin on a tax equivalent basis for the year ended December 31, 2025, was 3.22% compared with 2.93% for the same period in 2024. The changes to both measures were primarily due to a decrease in the average balances and average rates on other borrowings and a decrease in the average rates on interest-bearing deposits, partially offset by a decrease in the average balances and average rates on federal funds sold and other earning assets, a decrease in the average balances on investment securities, a decrease in the average rates on loans and a decrease in loan discount accretion of $5.1 million.

Noninterest income was $168.3 million for the year ended December 31, 2025, compared with $165.8 million for the same period in 2024, an increase of $2.5 million, primarily due to increases in other noninterest income and service charges on deposit accounts, partially offset by a decrease in net gain on sale or write-up of securities.

Noninterest expense was $556.2 million for the year ended December 31, 2025, compared with $570.6 million for the same period in 2024, a decrease of $14.4 million, primarily due lower regulatory assessments and FDIC insurance, a reversal of the 2024 FDIC special assessment, a decrease in other noninterest expense and a decrease in merger related expenses.

Balance Sheet Information

Prosperity had $38.463 billion in total assets at December 31, 2025, compared with $38.330 billion at September 30, 2025, and $39.567 billion at December 31, 2024. The year-over-year decrease was primarily due to the reduction in borrowings by $1.250 billion from December 31, 2024 to December 31, 2025.

Loans were $21.805 billion at December 31, 2025, a decrease of $222.4 million from $22.028 billion at September 30, 2025. Loans decreased $343.8 million from $22.149 billion at December 31, 2024.

Loans, excluding Warehouse Purchase Program loans, were $20.501 billion at December 31, 2025, compared with $20.750 billion at September 30, 2025, a decrease of $249.0 million, and compared with $21.068 billion at December 31, 2024, a decrease of $567.7 million.

Deposits were $28.482 billion at December 31, 2025, an increase of $700.4 million from $27.782 billion at September 30, 2025. Deposits increased $101.1 million from $28.381 billion at December 31, 2024.

Asset Quality

Nonperforming assets totaled $150.8 million or 0.46% of quarterly average interest-earning assets at December 31, 2025, compared with $119.6 million or 0.36% of quarterly average interest-earning assets at September 30, 2025, and $81.5 million or 0.23% of quarterly average interest-earning assets at December 31, 2024.

The allowance for credit losses on loans and off-balance sheet credit exposures was $371.4 million at December 31, 2025, compared with $377.3 million at September 30, 2025, and $389.5 million at December 31, 2024. There was no provision for credit losses for the three months and year ended December 31, 2025, compared with no provision for credit losses for the three months ended December 31, 2024, and a $9.1 million provision for credit losses related to acquisitions for the year ended December 31, 2024.

The allowance for credit losses on loans was $333.7 million or 1.53% of total loans at December 31, 2025, compared with $339.6 million or 1.54% of total loans at September 30, 2025, and $351.8 million or 1.59% of total loans at December 31, 2024. Excluding Warehouse Purchase Program loans, the allowance for credit losses on loans to total loans was 1.63%(1) at December 31, 2025, compared with 1.64%(1) at September 30, 2025, and 1.67%(1) at December 31, 2024.

Net charge-offs were $5.9 million for the three months ended December 31, 2025, compared with net charge-offs of $6.5 million for the three months ended September 30, 2025, and net charge-offs of $2.6 million for the three months ended December 31, 2024. For the three months ended December 31, 2025, $3.9 million of reserves on resolved purchased credit deteriorated (“PCD”) loans without any related charge-offs were released to the general reserve.

Net charge-offs were $18.1 million for the year ended December 31, 2025, compared with net charge-offs of $14.6 million for the year ended December 31, 2024. For the year ended December 31, 2025, $18.9 million of reserves on resolved PCD loans without any related charge-offs were released to the general reserve.

Dividend

Prosperity Bancshares declared a first quarter 2026 cash dividend of $0.60 per share to be paid on April 1, 2026, to all shareholders of record as of March 13, 2026.

Stock Repurchase Program

On January 26, 2026, Prosperity Bancshares announced a stock repurchase program under which up to 5%, or approximately 4.87 million shares, of its outstanding common stock may be acquired over a one-year period expiring on January 26, 2027, at the discretion of management. Under its 2025 stock repurchase program, Prosperity Bancshares repurchased approximately 2.04 million shares of its common stock at an average weighted price of $67.10 per share during the three months ended December 31, 2025, and approximately 2.34 million shares of its common stock at an average weighted price of $67.04 per share during the year ended December 31, 2025.

Pending Acquisition of Stellar Bancorp, Inc.

On January 28, 2026, Prosperity Bancshares and Stellar Bancorp, Inc. (“Stellar”) jointly announced the signing of a definitive merger agreement whereby Stellar, the parent company of Stellar Bank (“Stellar Bank”) will merge with and into Prosperity Bancshares. Stellar Bank operates 52 banking offices in greater Houston and Beaumont, Texas and surrounding areas. As of December 31, 2025, Stellar, on a consolidated basis, reported total assets of $10.807 billion, total loans of $7.301 billion and total deposits of $9.021 billion.

Under the terms and subject to the conditions of the definitive agreement, Prosperity Bancshares will issue 0.3803 shares of Prosperity Bancshares common stock and $11.36 in cash for each outstanding share of Stellar common stock. Based on Prosperity Bancshares’ closing price of $72.90 on January 27, 2026, the total consideration was valued at approximately $2.002 billion.

Pending Acquisition of Southwest Bancshares, Inc.

On October 1, 2025, Prosperity Bancshares and Southwest Bancshares, Inc. (“Southwest”) jointly announced the signing of a definitive merger agreement (the “Prosperity/Southwest Merger Agreement”) whereby Southwest, a Texas corporation and bank holding company of Texas Partners Bank (“Texas Partners”), will merge with and into Prosperity Bancshares and Texas Partners will merge with and into Prosperity Bank. Texas Partners operates 11 banking offices in Central Texas including its main office in San Antonio, and banking offices in the San Antonio area, Austin and the Hill Country. As of December 31, 2025, Southwest, on a consolidated basis, reported total assets of $2.426 billion, total loans of $1.941 billion and total deposits of $2.187 billion.

Under the terms and subject to the conditions of the Prosperity/Southwest Merger Agreement, Prosperity Bancshares will issue 4,062,520 shares of Prosperity Bancshares common stock for all outstanding shares of Southwest common stock and restricted stock awards, subject to certain potential adjustments. Southwest warrants and in-the-money Southwest stock options that are outstanding at the closing will be converted into cash payments based on the value of the merger consideration (less the applicable exercise price), as calculated pursuant to the terms of the Prosperity/Southwest Merger Agreement. Based on Prosperity Bancshares’ closing price of $65.97 on September 29, 2025, the total consideration was valued at approximately $268.9 million. Prosperity has received all necessary regulatory approvals for the acquisition of Southwest, and the shareholders of Southwest approved the transaction on January 22, 2026. The transaction is expected to become effective on February 1, 2026, subject to customary closing conditions.

Acquisition of American Bank Holding Corporation

On January 1, 2026, Prosperity completed the acquisition of American and its wholly owned subsidiary American Bank, headquartered in Corpus Christi, Texas. American Bank operated 18 banking offices and 2 loan production offices in South and Central Texas including its main office in Corpus Christi, and banking offices in San Antonio, Austin, Victoria and the greater Corpus Christi area including Port Aransas and Rockport and a loan production office in Houston, Texas. As of December 31, 2025, American, on a consolidated basis, reported total assets of $2.506 billion, total loans of $1.907 billion and total deposits of $2.271 billion.

Pursuant to the terms of the definitive agreement, Prosperity Bancshares issued 4,439,938 shares of Prosperity Bancshares common stock to the former shareholders and award holders of American in the first quarter of 2026.

Conference Call

Prosperity’s management team will host a conference call on Wednesday, January 28, 2026, at 11:30 a.m. Eastern Time (10:30 a.m. Central Time) to discuss Prosperity’s fourth quarter 2025 earnings and the Stellar acquisition announcement. Individuals and investment professionals may participate in the call by dialing 877-883-0383 for domestic participants, or 412-902-6506 for international participants. The participant elite entry number is 0259843.

Alternatively, individuals may listen to the live webcast of the presentation by visiting Prosperity’s website at www.prosperitybankusa.com. The webcast may be accessed from Prosperity’s Investor Relations page by selecting “Presentations, Webcasts & Calls” from the menu and following the instructions.

Non-GAAP Financial Measures

Prosperity’s management uses certain non-GAAP financial measures to evaluate its performance. Specifically, for internal planning and forecasting purposes, Prosperity reviews each of diluted earnings per share, return on average assets, return on average common equity, and return on average tangible common equity, in each case excluding merger related provision for credit losses, net of tax, merger related expenses, net of tax, FDIC special assessment, net of tax, and net gain on the sale or write-up of securities, net of tax; return on average tangible common equity; tangible book value per share; the tangible equity to tangible assets ratio; allowance for credit losses to total loans excluding Warehouse Purchase Program loans; the efficiency ratio, excluding net gains and losses on the sale, write-down or write-up of assets and securities; and the efficiency ratio, excluding net gains and losses on the sale, write-down or write-up of assets and securities, merger related expenses, and FDIC special assessment. Prosperity believes these non-GAAP financial measures provide information useful to investors in understanding Prosperity’s financial results and their presentation, together with the accompanying reconciliations, provides a more complete understanding of factors and trends affecting Prosperity’s business and allows investors to view performance in a manner similar to management, the entire financial services sector, bank stock analysts and bank regulators. Further, Prosperity believes that these non-GAAP financial measures provide useful information by excluding certain items that may not be indicative of its core operating earnings and business outlook. These non-GAAP financial measures should not be considered a substitute for, nor of greater importance than, GAAP basis financial measures and results; Prosperity strongly encourages investors to review its consolidated financial statements in their entirety and not to rely on any single financial measure. Because non-GAAP financial measures are not standardized, it may not be possible to compare these financial measures with other companies’ non-GAAP financial measures having the same or similar names. Please refer to the “Notes to Selected Financial Data” at the end of this Earnings Release for a reconciliation of these non-GAAP financial measures to the nearest respective GAAP financial measures.

Prosperity Bancshares, Inc. ®

As of December 31, 2025, Prosperity Bancshares, Inc.® is a $38.463 billion Houston, Texas based regional financial holding company providing personal banking services and investments to consumers and businesses throughout Texas and Oklahoma. Founded in 1983, Prosperity believes in a community banking philosophy, taking care of customers, businesses and communities in the areas it serves by providing financial solutions to simplify everyday financial needs. In addition to offering traditional deposit and loan products, Prosperity offers digital banking solutions, credit and debit cards, mortgage services, retail brokerage services, trust and wealth management, and treasury management.

Prosperity currently operates 301 full-service banking locations: 62 in the Houston area, including The Woodlands; 36 in the South Texas area including Corpus Christi and Victoria; 61 in the Dallas/Fort Worth area; 22 in the East Texas area; 28 in the Central Texas area including Austin and San Antonio; 45 in the West Texas area including Lubbock, Midland-Odessa, Abilene, Amarillo and Wichita Falls; 15 in the Bryan/College Station area; 6 in the Central Oklahoma area; and 8 in the Tulsa, Oklahoma area and 18 in the Central, South Texas and San Antonio areas currently doing business as American Bank.

Cautionary Notes on Forward-Looking Statements

“Safe Harbor” Statement under the Private Securities Litigation Reform Act of 1995: This press release contains, and the remarks by Prosperity’s management on the conference call may contain, statements regarding the proposed transaction between Prosperity Bancshares, Inc. (“Prosperity”) and Stellar Bancorp, Inc. (“Stellar”); future financial and operating results; benefits and synergies of the proposed transaction; future opportunities for Prosperity; the issuance of common stock of Prosperity contemplated by the Agreement and Plan of Merger by and between Prosperity and Stellar (the “Merger Agreement”); the expected filing by Prosperity with the Securities and Exchange Commission (the “SEC”) of a registration statement on Form S-4 (the “Registration Statement”) and a prospectus of Prosperity and a proxy statement of Stellar to be included therein (the “Proxy Statement/Prospectus”); the expected timing of the closing of the proposed transaction; the ability of the parties to complete the proposed transaction considering the various closing conditions and any other statements about future expectations that constitute forward-looking statements within the meaning of the federal securities laws, including the meaning of the Private Securities Litigation Reform Act of 1995, as amended, Section 27A of the Securities Act of 1933, as amended (the “Securities Act”), and Section 21E of the Securities Exchange Act of 1934, as amended. From time to time, oral or written forward-looking statements may also be included in other information released to the public. Such forward-looking statements are typically, but not exclusively, identified by the use in the statements of words or phrases such as “aim,” “anticipate,” “believe,” “estimate,” “expect,” “goal,” “guidance,” “intend,” “is anticipated,” “is expected,” “is intended,” “objective,” “plan,” “projected,” “projection,” “will affect,” “will be,” “will continue,” “will decrease,” “will grow,” “will impact,” “will increase,” “will incur,” “will reduce,” “will remain,” “will result,” “would be,” variations of such words or phrases (including where the word “could,” “may,” or “would” is used rather than the word “will” in a phrase) and similar words and phrases indicating that the statement addresses some future result, occurrence, plan or objective. Forward-looking statements include all statements other than statements of historical fact, including forecasts or trends, and are based on current expectations, assumptions, estimates, and projections about Prosperity, Stellar and their respective subsidiaries or related to the proposed transaction between Prosperity and Stellar and are subject to significant risks and uncertainties that could cause actual results to differ materially from the results expressed in such statements.

These forward-looking statements may include information about Prosperity’s and Stellar’s possible or assumed future economic performance or future results of operations, including future revenues, income, expenses, provision for loan losses, provision for taxes, effective tax rate, earnings per share and cash flows and Prosperity’s and Stellar’s future capital expenditures and dividends, future financial condition and changes therein, including changes in Prosperity’s and Stellar’s loan portfolio and allowance for loan losses, future capital structure or changes therein, as well as the plans and objectives of management for Prosperity’s and Stellar’s future operations, future or proposed acquisitions, the future or expected effect of acquisitions on Prosperity’s and Stellar’s operations, results of operations, financial condition, and future economic performance, statements about the anticipated benefits of the proposed transaction, and statements about the assumptions underlying any such statement.

These forward-looking statements are not guarantees of future performance and are based on expectations and assumptions Prosperity and Stellar currently believe to be valid. Because forward-looking statements relate to future results and occurrences, many of which are outside of the control of Prosperity and Stellar, they are subject to inherent uncertainties, risks and changes in circumstances that are difficult to predict. These risks and uncertainties include, but are not limited to, whether Prosperity can: successfully identify acquisition targets and integrate the businesses of acquired companies and banks; continue to sustain its current internal growth rate or total growth rate; provide products and services that appeal to its customers; continue to have access to debt and equity capital markets; and achieve its sales objectives. Other risks include, but are not limited to: the possibility that credit quality could deteriorate; actions of competitors; changes in laws and regulations (including changes in governmental interpretations of regulations and changes in accounting standards); a deterioration or downgrade in the credit quality and credit agency ratings of the securities in Prosperity’s securities portfolio; customer and consumer demand, including customer and consumer response to marketing; effectiveness of spending, investments or programs; fluctuations in the cost and availability of supply chain resources; economic conditions, including currency rate, interest rate and commodity price fluctuations; changes in trade policies by the United States or other countries, such as tariffs or retaliatory tariffs; and the effect, impact, potential duration or other implications of weather and climate-related events. Many possible events or factors could adversely affect the future financial results and performance of Prosperity, Stellar or the combined company and could cause those results or performance to differ materially from those expressed in or implied by the forward-looking statements. Such risks and uncertainties include, among others: (1) the risk that the cost savings and synergies from the proposed transaction may not be fully realized or may take longer than anticipated to be realized, (2) disruption to Prosperity’s and Stellar’s businesses as a result of the announcements and pendency of the proposed transaction, (3) the risk that the integration of Stellar’s businesses and operations into Prosperity will be materially delayed or will be more costly or difficult than expected, or that Prosperity is otherwise unable to successfully integrate Stellar’s business into its own, including as a result of unexpected factors or events, (4) the failure to obtain the necessary approval by the shareholders of Stellar, (5) the ability by Prosperity and/or Stellar to obtain required governmental approvals of the proposed transaction on the timeline expected, or at all, and the risk that such approvals may result in the imposition of conditions that could adversely affect Prosperity after the closing of the proposed transaction or adversely affect the expected benefits of the proposed transaction, (6) reputational risk and the reaction of each company’s customers, suppliers, employees or other business partners to the proposed transaction, (7) the failure of the closing conditions in the Merger Agreement to be satisfied, or any unexpected delay in closing the proposed transaction or the occurrence of any event, change or other circumstances

that could give rise to the termination of the Merger Agreement, (8) the dilution caused by the issuances of additional shares of Prosperity’s common stock in the proposed transaction, (9) the possibility that the proposed transaction may be more expensive to complete than anticipated, including as a result of unexpected factors or events, (10) the outcome of any legal or regulatory proceedings that may be currently pending or later instituted against Prosperity before or after the proposed transaction, or against Stellar, (11) diversion of management’s attention from ongoing business operations and (12) general competitive, economic, political and market conditions and other factors that may affect future results of Prosperity and Stellar. Prosperity and Stellar disclaim any obligation to update such factors or to publicly announce the results of any revisions to any of the forward-looking statements included herein to reflect future events or developments. These and various other risks, uncertainties, assumptions, and factors are discussed in the respective Annual Reports on Form 10-K for the year ended December 31, 2024, Quarterly Reports on Form 10-Q, and Current Reports on Form 8-K, filed by Prosperity or Stellar and in other filings made by Prosperity and Stellar with the SEC from time to time.

Additional Information about the Transaction and Where to Find It

Prosperity intends to file with the SEC the Registration Statement on Form S-4 to register the shares of Prosperity common stock to be issued to the shareholders of Stellar in connection with the proposed transaction. The Registration Statement will include the Proxy Statement/Prospectus which will be sent to the shareholders of Stellar in connection with the proposed transaction. This communication is not a substitute for the Registration Statement, the Proxy Statement/Prospectus or any other document that may be filed by Prosperity or Stellar with the SEC. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE REGISTRATION STATEMENT, THE PROXY STATEMENT/PROSPECTUS AND ANY OTHER RELEVANT DOCUMENTS TO BE FILED WITH THE SEC IN CONNECTION WITH THE PROPOSED TRANSACTION OR INCORPORATED BY REFERENCE INTO THE PROXY/STATEMENT PROSPECTUS, AS WELL AS ANY AMENDMENTS OR SUPPLEMENTS TO THESE DOCUMENTS, CAREFULLY AND IN THEIR ENTIRETY, BECAUSE THEY CONTAIN OR WILL CONTAIN IMPORTANT INFORMATION. Investors and security holders will be able to obtain the Registration Statement and the Proxy Statement/Prospectus (when available) and other documents that are filed with the SEC by Prosperity or Stellar, as applicable, free of charge from the SEC’s website at https://www.sec.gov or through the investor relations section of Prosperity’s website at https://www.prosperitybankusa.com/investor-relations/ or Stellar’s website at https://ir.stellar.bank.

Participants in the Solicitation

Prosperity, Stellar and certain of their directors and executive officers and other employees may be deemed to be participants in the solicitation of proxies from Stellar’s shareholders in connection with the proposed transaction. Information about the directors and executive officers of Prosperity and their ownership of Prosperity common stock is contained in the definitive proxy statement for Prosperity’s 2025 annual meeting of shareholders (the “Prosperity Annual Meeting Proxy Statement”), which was filed with the SEC on March 13, 2025, including under the headings “Item 1. Election of Directors,” “Corporate Governance,” “Executive Compensation and Other Matters,” “Item 3. Advisory Vote on Executive Compensation,” and “Beneficial Ownership of Common Stock by Management of the Company and Principal Shareholders.” Information about the directors and executive officers of Stellar and their ownership of Stellar common stock is contained in the definitive proxy statement for Stellar’s 2025 annual meeting of shareholders (the “Stellar Annual Meeting Proxy Statement”), which was filed with the SEC on April 10, 2025, including under the headings “Proposal 1: Election of Directors,” “Certain Corporate Governance Matters,” “Executive Compensation and Other Matters,” “Executive Compensation Payments and Awards,” “Proposal 4: Advisory Vote on the Compensation of the Company’s Named Executive Officers (“Say-on-Pay Resolution”),” and “Beneficial Ownership of the Company’s Common Stock by Management and Principal Shareholders of the Company.” Additional information regarding the persons who may, under the rules of the SEC, be deemed participants in the solicitation of the shareholders of Stellar in connection with the proposed transaction, including a description of their direct or indirect interests, by security holdings or otherwise, will be included in the Proxy Statement/Prospectus relating to the proposed transaction when it is filed with the SEC. To the extent holdings of securities by potential participants (or the identity of such participants) have changed since the information printed in the Prosperity Annual Meeting Proxy Statement or the Stellar Annual Meeting Proxy Statement, such information has been or will be reflected on Statements of Change in Ownership on Forms 3 and 4 filed with the SEC, as applicable. Free copies of the Proxy Statement/Prospectus relating to the proposed transaction and free copies of the other SEC filings to which reference is made in this paragraph may be obtained from the SEC’s website at https://www.sec.gov or through the investor relations section of Prosperity’s website at https://www.prosperitybankusa.com/investor-relations/ or Stellar’s website at https://ir.stellar.bank.

No Offer or Solicitation

This communication is for informational purposes only and is not intended to and does not constitute an offer to subscribe for, buy or sell, or the solicitation of an offer to subscribe for, buy or sell, or an invitation to subscribe for, buy or sell any securities or a solicitation of any vote or approval in any jurisdiction, nor shall there be any sale, issuance or transfer of securities in any jurisdiction in which such offer, invitation, sale or solicitation would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act, and otherwise in accordance with applicable law.

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(A) Includes ($375), ($1,987), ($1,657), ($1,374) and ($2,056) in unrealized losses on available for sale securities for the quarterly periods ended December 31, 2025, September 30, 2025, June 30, 2025, March 31, 2025, and December 31, 2024, respectively.

(B) Includes ($296), ($1,570), ($1,309), ($1,085) and ($1,624) in after-tax unrealized losses on available for sale securities for the quarterly periods ended December 31, 2025, September 30, 2025, June 30, 2025, March 31, 2025, and December 31, 2024, respectively.